1. Field of the Invention
The present invention relates to a device for detecting a state of liquid droplets discharged from a nozzle of a recording head, and an image-forming apparatus incorporating such a device.
2. Description of the Related Art
Serial type image-forming apparatuses and line type image-forming apparatuses are well known. The serial type image-forming apparatus is configured to discharge liquid droplets toward a recording paper from a nozzle of a recording head while moving the recording head in a main-scanning direction which is orthogonal to a sub-scanning direction of a feeding direction of the recording paper as a recording medium. The line type image-forming apparatus is configured to discharge liquid droplets toward a recording paper from nozzles of a plurality of recording heads fixed in the main-scanning direction while feeding the recording paper in the sub-scanning direction.
In these types of image-forming apparatuses, a discharge error of the nozzle may occur due to an increase in ink viscosity and ink solidification arising from evaporation of solvent from a nozzle hole, dust attachment to the nozzle hole, bubble interfusion into ink or the like. Image quality is degraded when such a discharge error occurs.
In view of this, these types of image-forming apparatuses are provided with a device for detecting a state of liquid droplets discharged from a recording head (refer to Patent Document 1: JP4925184B). This device determines the presence or absence of a discharged liquid droplet, and includes a light emitter which is provided on one side of a recording paper in the width direction orthogonal to the feeding direction of the recording paper, and a light receiver which is provided on the other side of the recording paper in the width direction.
The light emitter includes a light-emitting element, collimator lens, and aperture stop member. A square aperture opening is formed in the aperture stop member.
Light from the light-emitting element is condensed by the collimator lens, and passes through the aperture opening of the aperture stop member as light beams to irradiate the light receiver. A light-receiving surface of the light receiver is displaced in the feeding direction of a paper relative to the optical axis (optical axis of light-receiving element) of the light emitter.
This device receives scattered light of light beams with the light receiver, which scatters forward in the traveling direction of the light beams due to the existence of the liquid droplets, to thereby determine presence or absence of a liquid droplet by the output of the light receiver. Also, this device reduces the attachment of the mist, which is generated upon the discharge of liquid droplets, to the light-emitting element and the collimator lens.
In the device described in the above document, when the light beams pass through the aperture opening, diffracted light with the light beams is generated in the direction of the line segments connecting respective middle points of the parallel two sides of the aperture opening.
For this reason, when the light-receiving surface of the light receiver is displaced in the direction of the line segment connecting the respective middle points of the two sides, the amount of diffracted light which is incident on the light-receiving surface as offset light increases, noise relative to the amount of scattered light due to liquid droplets relatively increases, and the detection accuracy of the state of the discharged liquid droplets is lowered due to an increase in S/N ratio.